Non-insulin-dependent Diabetes Mellitus ("NIDDM"), also known as Type II diabetes, is a debilitating disease characterized by high circulating blood glucose, insulin and corticosteroid levels. Increased hepatic glucose production, or gluconeogenesis, is the primary factor responsible for loss of glycemic control and leads to hyperglycemia and hyperinsulinemia. In individuals having Type II diabetes, excess glucose production occurs in spite of the availability of insulin, and circulating glucose levels remain excessively high as a result of inadequate glucose clearance. Syndrome X is a condition characterized by many of the same symptoms, and is generally a reliable early indicator of the development of Type II Diabetes.
The adrenal cortex synthesizes and releases three classes of steroid hormones: glucocorticoids; the sex steroids; and mineralocorticoids. Secretion of glucocorticoids and mineralocorticoids is known to follow a circadian pattern. The adrenal cortical hormones are lipid-soluble and readily pass through cell membranes of target tissues into the cytoplasm, where they combine with specific intracellular receptor proteins. Hormone-receptor complexes are translocated to the nucleus, where they bind to a glucocorticoid response element (GRE), which selectively activates or represses transcription from specific promoters.
In particular, glucocorticoids regulate the transcription of a number of genes which regulate gluconeogenesis, particularly PEPCK (Friedman et al., (1993), J. Biol. Chem., V268, p12952). PEPCK catalyzes the conversion of oxaloacetate to phosphoenolpyruvate and is considered a key regulatory step in gluconeogenesis. PEPCK activity and corresponding mRNA levels are elevated in NIDDM models. Unlike many enzymes in regulatory metabolic pathways, PEPCK is regulated primarily by hormonally induced changes in gene transcription.
Cortisol is the most important human glucocorticoid, and is synthesized endogenously through a series of reactions resulting in the conversion of cholesterol to cortisol. The comparable rodent glucocorticoid is corticosterone. Cortisol promotes gluconeogenesis and glycogen deposition in the liver, increases blood glucose levels, and decreases peripheral utilization of glucose. It also stimulates utilization of fatty acids and ketogenesis and has weak mineralocorticoid activity. Mineralocorticoids promote the retention of Na.sup.+ and the loss of K.sup.+ by the kidneys and thus assist in maintaining water and salt balances in the body. The major mineralocorticoid is aldosterone, which also has weak glucocorticoid activity.
Glucocorticoid receptors belong to a large super-family of ligand-dependent transcription factors that play diverse roles in homeostasis, growth and development. Two types of intracellular receptors bind corticosteroids with high affinity: glucocorticoid receptor type I, also referred to as mineralocorticoid receptor ("MR"); and glucocorticoid receptor type II, also referred to as glucocorticoid receptor ("GR"). Both the type I and type II receptors are activated by the same endogenous ligands and, in some cases, may regulate expression of the same genes. The type I receptor binds aldosterone and corticosterone with approximately equal affinity, and dexamethasone with lower affinity. The type II receptor binds dexamethasone with high affinity, and aldosterone and corticosterone with lower affinity. Coexpression of the enzyme 11-.beta. hydroxysteroid dehydrogenase with the type I receptor metabolizes cortisol to its inactive form, cortisone, ensuring that the mineralocorticoids preferentially occupy the receptor. 11-.beta. hydroxysteroid dehydrogenase is not present in the central nervous system, and glucocorticoids consequently bind to the type I receptor with high affinity in the central nervous system.
Type I and type II glucocorticoid receptors are found both centrally and peripherally. Type II receptor expression is moderate in the cerebral cortex, amygdala, thalamnus and hypothalamus, and abundant in the septum and hippocampus. The type I receptor has a limited distribution with moderate to high levels in the septal area and hippocainpus. Peripherally, type II receptors are expressed predominantly by adipose and liver cells, while Type I receptors are not normally accessible to glucocorticoid binding.
Certain functional domains within the receptor molecules have been identified that are thought to be responsible for DNA binding, hormone binding, and nuclear localization. The ligand binding domain has the ability to block activity of the receptor in the absence of hormone and thus, presence of the requisite hormone relieves the inhibition of the receptor to activity. Numerous hormone and hormone-like receptors have been isolated, identified, characterized and prepared.
Glucocorticoids are known to play an important role in the development and maintenance of obesity. The importance of glucocorticoids in glycemic control was established by showing that adrenalectomized diabetic mice returned to normal glycemia. Glucocorticoids are generally thought to play a permissive role by enhancing the availability of gluconeogenic substrates and increasing the sensitivity of the liver to the actions of glucagon and catecholamines.
Several studies have implicated the type II glucocorticoid receptor in the obese phenotype. A study using adrenalectomized gold thioglucose (GTG)-obese mice demonstrated that icv administration of the selective type II glucocorticoid agonist dexamethasone restored the obese phenotype, while type I agonism in contrast, using icv administration of desoxycorticosterone, lead to exasperated weight loss and eventually to death. A. F. Debons et al. "Gold thioglucose-induced hypothalamic damage, hyperphagia, and obesity: dependence on the adrenal gland"; Endocrinology 110:2024-2029, (1982). Another study demonstrated that icv administration of dexamethasone reduced thermogenesis in adrenalectomized ob/ob mice, while icv administration of aldosterone (a selective type I agonist) was without effect on thermogenesis. Chen, H. L. and Romsos, D. R., "Type II glucocorticoid receptors in the CNS regulate metabolism in ob/ob mice independent of protein synthesis," Am. J. Physiol. 1994 Mar;266 (3 Pt. 1): E427-32. Yet another study involved treating adrenalectomized ob/ob mice with either cortisone or desoxycortisone and demonstrated that only cortisone reduced lean muscle weight, while also increasing food intake and adipose weight. M. Saito and G. F. Bray, "Adrenalectomy and food restriction in the genetically obese (ob/ob) mouse," Am. J. Physiol. 246:R20-R25, (1984). Okada et al. (Am. J. Physiol., (1992), 272, p. R106) showed that mifepristone (RU486), a glucocorticoid receptor type II antagonist, reverses a dietary form of obesity. When Osbome-Mendel (OM) rats were placed on a high fat diet, they gained more weight and had larger retroperitoneal and parametrial fat pads than OM rats fed high-carbohydrate low fat diet. RU486 (30mg/kg-day) for 14 days completely reversed the body weight gain and the increase in fat pad size for the OM rats on the high fat diet. The authors suggested that the type II glucocorticoid receptor modulates body fat deposition and is essential for the development of obesity.